Increasing lower level monster experience gain

I’m noticing even if a monster is 15-20 levels below the rest of my party, they really don’t level up to equal the rest very quickly. Even monsters I’ve had for a hundred battles are still a few levels behind ones I’ve stuck with for a long time. It seems like creatures that are a lower level don’t really get a particular experience bonus, but the experience does go down if your level is too high - I’ve seen level 35 and 17 monsters gain the same amount from a fairly high level battle. Is it possible this can be tweaked so the lower level monsters get more of a bonus so they even out quicker?